Search

The more sensitive the observations, the more complex the gas in our Galaxy appears. Since the detection of neutral hydrogen in 1951, each new survey has revealed new structure. Using the GALFA-HI survey, we have discovered five populations of compact neutral hydrogen clouds. We began by developing a machine-vision algorithm to identify compact clouds in the GALFA-HI Data Release 1. Based on position, velocity, and linewidth we separated the 1964 identified clouds into five populations: galaxy candidates, high-velocity clouds, cold low-velocity clouds, warm low-velocity clouds, and warm positive low-velocity clouds in the third Galactic quadrant. We found that the dust properties of the compact clouds support our population definitions. Using both IRAS and the newly released Planck data, we found no dust detections in the high-velocity clouds, or the warm positive low-velocity clouds in the third Galactic quadrant. We claim that the third quadrant clouds are low-velocity halo clouds. The warm low-velocity clouds have a significantly greater dust-to-gas ratio than the cold low-velocity clouds. We interpret this as evidence that the warm clouds have an ionized component not present with the cold clouds, possibly because they are part of the Galactic fountain.

We present a search for radio transients in the field of the bright radio source 3C 286 using archival observations from the Very Large Array. These observations span 23 years and include 1852 epochs at 1.4 GHz in the C and D configurations. We find no transients in the field. The sensitivity of the observations is limited by dynamic range effects in the images. At large flux densities ($> 0.2$ Jy), single epoch observations provide a strong limit on the transient surface density. At flux densities near the dynamic range threshold, we use the requirement that transient sources must appear in consecutive epochs to be confirmed as real. This sets the sensitivity at low flux densities to transient durations of $\tau \sim 1$ day, while $\tau > 1$ minute for high flux densities. At 70 mJy, we find a 1-$\sigma$ limit on the surface density $\Sigma < 3 \times 10^{-3}$ deg$^{-2}$. At 3 Jy, we find a 1-$\sigma$ limit $\Sigma < 9 \times 10^{-4}$ deg$^{-2}$. A future systematic search of the VLA archives can provide one to two orders of magnitude more sensitivity to radio transients., Comment: Accepted for ApJL

The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR), at a velocity resolution of 0.18 km/s and an angular resolution of 3.5 arcmin. The unprecedented resolution and sensitivity of the GALFA-HI survey have resulted in the detection of many isolated, very compact HI clouds at low Galactic velocities which are distinctly separated from the HI disk emission. In the limited area of ~4600 deg$^2$ searched so far, we have detected 96 such compact clouds. The detected clouds are cold with kinetic temperature less than 300 K. Moreover, they are quite compact and faint, with median values of 5 arcmin in angular size, 0.75 K in peak brightness temperature, and $5\times10^{18}$ cm$^{-2}$ in HI column density. From the modeling of spatial and velocity distributions of the whole compact cloud population, we find that the bulk of clouds are related to the Galactic disk, and are within a few kpc distance. We present properties of the compact clouds sample and discuss various possible scenarios for the origin of this clouds population and its role in the Galactic interstellar medium studies., Comment: 7 pages, 3 figures. To appear in "The Dynamic ISM: A celebration of the Canadian Galactic Plane Survey," ASP Conference Series